Pressure swing adsorption is a well known method of separating the components of a gaseous mixture by passage through a bed of adsorbent that preferentially adsorbs at least one component. A gaseous product that is relatively lean in the adsorbed component(s) passes out of the bed. The bed is regenerated by subjecting it to a lower pressure thereby desorbing the previously adsorbed component(s). The adsorbent is generally a molecular sieve, e.g. a zeolite or carbon molecular sieve. In more efficient commercial PSA processes, a plurality of beds is employed and the incoming gas stream for separation is switched between the beds so as to facilitate the continuous supply of gaseous products. Pressure swing adsorption processes are for example described in our UK patent applications 2073043A and 2163669A, and an improved apparatus for separation of gaseous mixture by pressure swing adsorption is described in our UK patent application 2163670A.
The equilibrium quantity of a gas adsorbed on a molecular sieve is not solely a function of pressure but also one of temperature. Indeed, some commercial gas separation processes effect separation by temperature swing rather than pressure swing. Although typical zeolite molecular sieves have gaseous adsorption equilibrium values that are achieved rapidly and then remain constant with time, carbon molecular sieves exhibit dynamic sieving behavior before coming to equilibrium (the former effects the separation), both kinds of sieve increase in temperature as gas is adsorbed since heat of adsorption is liberated, and decrease in temperature again when gas is desorbed. These changes in temperature are substantially equal. There is, however, an additional increase in temperature as a result of the compression of the incoming gas mixture for separation. A substantial proportion of the heat of compression is removed in an after cooler that is conventionally associated with the compressor. There is also a reduction in temperature associated with the reduction in pressure during the desorption step. It might be expected that the PSA process would therefore run at an average temperature below ambient in view of there being net refrigeration that is produced by the pressure reduction required to effect the desorption step. In practice, however, only a relatively small proportion of the refrigeration developed during the desorption step is employed to reduce the temperature of the bed of adsorbent, and most of the refrigeration generated during desorption is wasted in the gas that is vented to the atmosphere. Thus, in practice, the average temperature at which the pressure swing adsorption process operates is usually above rather than below ambient temperature. Since the equilibrium amount of gas that is adsorbed increases with decreasing temperature, the failure to efficiently use the refrigeration generated leads to unnecessarily high specific power consumption. Moreover, the temperature rise that takes place during adsorption is also undesirable since lower temperatures generally favor adsorption. The temperature fall that takes place during desorption is similarly undesirable since in general higher temperatures favor desorption.
There is thus a need to create a more favorable thermal regime during a pressure swing adsorption process and it is an aim of the present invention to provide a method and apparatus for meeting this need.